The annual mean limit value is 40 µg NO2/m³. Over the years 1997-2008 the total population, for which exposure estimates are made, increased from 55 to 118 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Bosnia and Herzegovina, Iceland, Liechtenstein, Luxembourg, Malta, the former Yugoslav Republic of Macedonia, Serbia, and Turkey are not included due to the geographical coverage of the Urban Audit and/or lack of air quality data.

The data presented were derived from a consistent set of stations in all years.
Statistically significant trends (level of significance 0.1) are calculated by applying the Mann-Kendall test. Increasing concentrations are indicated with red dots and decreasing concentrations with green dots, when statistically significant. The applied method is described in de Leeuw, 2012.

The NO2 monitoring data in AirBase provide the
basis for estimating the exposure of the European
population to exceedances of the NO2 annual mean
limit value of 40 µg/m3. Figure 4.4 presents this data
for the period 2002–2011, based on NO2 measured
at urban background monitoring stations.

The graphs are based on the 90.4 percentile of daily mean concentration values corresponding to the 36th highest daily mean; they present the range of concentrations at all station types (in μg/m3) officially reported by the EU Member States and how the concentrations relate to the limit value set by EU legislation (marked by the red line).
The diagram indicates the lowest and highest observations, the means and the lower and upper quartiles. The lower quartile splits the lowest 25 % of the data and the upper quartile splits the highest 25 % of the data.

The annual mean limit value is 40 µg NO2/m3 and to be met by 2010. Over the years 2001-2010 the total population, for which exposure estimates are made, increased from 92 to 116 million people due to an increasing number of monitoring stations reporting air quality data under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.

Eutrophication The magnitude of the risk of ecosystem eutrophication and its geographical coverage has diminished only slightly over the years. The predictions for 2010 and 2020 indicate that the risk is still widespread over Europe. This is in conflict with the EU's long-term objective of not exceeding critical loads of airborne acidifying and eutrophying substances in sensitive ecosystem areas (National Emission Ceilings Directive, 6th Environmental Action Programme, Thematic Strategy on Air Pollution).
Acidification The situation has considerably improved and it is predicted to improve further. The interim environmental objective for 2010 (National Emission Ceilings Directive) will most likely not be met completely. However, the European ecosystem areas where the critical load will be exceeded is predicted to have declined by more than 80 % in 2010 with 1990 as a base year. By 2020, it is expected that the risk of ecosystem acidification will only be an issue at some hot spots, in particular at the border area between the Netherlands and Germany.
Ozone (O 3 ) Most vegetation and agricultural crops are exposed to ozone levels exceeding the long term objective given in the EU Air Quality Directive. A significant fraction is also exposed to levels above the 2010 target value defined in the Directive. Concentrations in 2009 were on the average lower than in 2008. The effect-related accumulated concentrations, addressing exposure of crops to ozone over several summer months, shows large year-to-year variations. Over the period 1996-2009 there is a tendency to increased exposure, although this development has not proven to be statistically significant.